In their efforts to meet the demands of today's competitive and worldwide market structures, producers of paper pulp are confronted with stringent requirements for their pulp product to be of a high, consistent and well-documented quality. Pulp producers use a number of optical and physical properties to specify the grade of their product. Tests according to established industry standards are commonly used to determine various pulp properties, such as brightness, opacity, bulk, shive content, elasticity, tensile strength, moisture etc.
A pulp brightness test is a test which provides an indication of a reflectance property of a pulp sample. More particularly, a pulp brightness test is typically a test which provides an indication of the reflectance of blue light by the pulp. A pulp brightness test may be conducted using a number of different methods and according to a number of different standards.
For example, a pulp brightness test may be conducted as a directional brightness test in which a light source comprising a single light is directed at a pulp sample at a specific angle and the reflectance of the pulp is measured as the light is reflected from the pulp. A standard relating to a directional brightness test is the Technical Association for the Pulp, Paper and Converting Industry (TAPPI) standard T 452.
Alternatively, a pulp brightness test may be conducted as a diffuse brightness test in which a light source comprising one or more lights is directed within a diffusing device such as an integrating sphere and is caused to reflect within the diffusing device before reaching the pulp sample. Standards relating to diffuse brightness tests include TAPPI standard T 525, International Standards Organization (ISO) standard ISO 2469 and ISO standard ISO 2470.
Pulp brightness according to ISO standard ISO 2470 is characterized by a diffuse blue reflectance factor of the pulp, which is defined as the amount of diffuse blue light reflected by the pulp (i.e., reflectance of the pulp) in a given direction expressed as a percentage of the reflectance by a perfect reflecting diffuser under the same conditions. Pulp brightness according to ISO standard ISO 2470 is therefore often referred to as the “diffuse blue reflectance factor” or “ISO brightness”.
Currently, pulp producers evaluate pulp brightness by manually sampling pulp bales on a pulp finishing line and conducting one or more of the standard tests. Drawbacks inherent to this approach include obvious sample limitations, as well as time delays. For example, it may be estimated that the quality of sixty bales of pulp could conceivably be assessed by using only six grams of a pulp sample. Furthermore, three to four hours are needed to obtain the results of the quality tests.
In view of the shortcomings associated with the traditional methods of assessing the properties of pulp samples, various methods have been proposed to circumvent the intrinsic problems associated with the standard tests. For example, the prior art teaches different approaches to on-line monitoring of the quality of pulp samples.
Trotter, A. B. et al “An On-Line Pulp Bale Brightness Meter for Pulp Bale Information Systems” (Proceedings of the 47th Appita Annual General Conference; Rotorua, New Zealand; 1993, Vol. 2, p. 651-655) describes a brightness meter for measuring the brightness of pulp bales, and a method for on-line measurements of pulp brightness. Like the lab pulp brightness meter (based on ISO standard ISO 2469 and ISO 2470), a measuring head is built around an integrating sphere to provide diffuse light source for the measurement. For on-line operation the measuring head is mounted on a platform that is raised and lowered by two air pistons. The whole unit is housed in a rectangular box and constructed so that the box opens by rotating or pivoting a covering part of the box. A lid portion of the covering part carries a reflectance tile on its underside which, as well as covering the measuring port when the box is closed, allows the system to check its response against a tile of known reflectance.
Morgan, Stephen and Jeune, Max, “The Development and Mill Application of a Real Time, On-Line Pulp Dirt Sensor” (Proceedings of the Wastepaper VII Conference; Chicago, Ill., U.S.A., 1996) describes a system for detecting specks and dirt particles in a paper or pulp sheet.
Nilsson, C. M. et al, “Application of Optical Spectroscopy to Paper Production” (SPIE Vol. 3824, p. 318-325) describes the application of fluorescence monitoring to study the relative shrinking of paper during drying.
Malmström, E., “Quality Measurement in Real Time has Become a Reality” Svensk Papperstidn., Vol. 102, No. 9, p. 38-39 (1999) describes a system for measuring the properties of pulp in real time which combines NIR-spectroscopy with multivariate data examination, but does not appear to utilize reflectance measurements.
U.S. Patent Application Publication No. US 2003/0155092 A1 (Badenlid et al) describes a method for predicting properties of a product that consists of cellulose-fiber-based pulp, paper or paperboard by means of spectroscopic measurements in a selected spectrum in the wavelength range of 200-25000 nm. In the method, a sample quantity of fiber is extracted and diluted prior to analysis, following which one partial flow of the diluted sample quantity is dewatered, dried and used for spectroscopic measurements, while a second partial flow of the diluted sample quantity is used for analysis of physical fiber data by means of image analysis. The sample quantity may be extracted for analysis at-line or on-line in a process line.
U.S. Pat. No. 4,040,743 (Villaume et al) describes a method and an apparatus for measuring the brightness and/or consistency of a pulp slurry. The apparatus is comprised of an optical probe which introduces light energy into the pulp slurry. The light energy is transmitted, back-scattered and reflected orthogonally. Three signals are provided which correspond to the transmitted, back-scattered and reflected light energy. The signals are ratioed against a reference signal which corresponds to the intensity of the light energy introduced into the pulp slurry to provide three outputs proportional to the difference between the difference between the reference signal and the provided signals. The outputs are used to provide independent measurements of brightness of the pulp slurry and consistency of the pulp slurry.
U.S. Pat. No. 5,792,942 (Hosokawa) describes an apparatus for determining properties of a slurry material such as sludge and pulp slurry which is adapted to obtain samples for analysis directly out of a slurry transfer passage or a slurry storage tank.
There remains a need for an apparatus and a method for obtaining a reflectance property indication of a lignocellulosic material which provides an alternative to the apparatus and methods disclosed in the prior art.